This invention relates to a free radical polymerization process with characteristics of a living polymerization system in that it is capable of producing polymers of pre-determined molecular weight with narrow molecular weight distribution (low polydispersity), and, by successively adding different monomers, can be used to make block polymers. The process can be used to produce polymers of more complex architecture, including variously branched homo- and copolymers. The use of certain reagents in this process and the polymers produced thereby are also claimed. Novel chain transfer agents for use in the process are also claimed.
There is increasing interest in methods for producing a variety of polymers with control of the major variables affecting polymer properties. Living polymerizations provide the maximum degree of control for the synthesis of polymers with predictable well defined structures. The characteristics of a living polymerization are discussed by Quirk and Lee (Polymer International 27, 359 (1992)) who give the following experimentally observable criteria:                “1. Polymerization proceeds until all of the monomer has been consumed. Further addition of monomer results in continued polymerization.        2. The number average molecular weight (or the number average degree of polymerization) is a linear function of conversion.        3. The number of polymer molecules (and active centers) is a constant which is sensibly independent of conversion.        4. The molecular weight can be controlled by the stoichiometry of the reaction.        5. Narrow molecular weight distribution polymers are produced.        6. Block copolymers can be prepared by sequential monomer addition.        7. Chain end-functionalized polymers can be prepared in quantitative yield.”        
Living polymerization processes can be used to produce polymers of narrow molecular weight distribution containing one or more monomer sequences whose length and composition are controlled by the stoichiometry of the reaction and the degree of conversion. Homopolymers, random copolymers or block polymers can be produced with a high degree of control and with low polydispersity. Swarc (Adv. Polym. Sci. 49, 1 (1983)) stated that living polymerization to give polymers of narrow molecular weight distribution requires the absence of chain transfer and termination reactions, the elementary reactions being only initiation and propagation, which take place uniformly with respect to all growing polymer chains. Later Inoue and Aida in an article on living polymer systems (Encyclopedia of Polymer Science and Engineering, Supplement Volume, Wiley Interscience New York 1989) stated “If chain transfer and terminating agents are present in the polymerization system the living character of the polymerization is lost, and the formation of polymer with narrow molecular weight distribution does not result.”
However, it has been shown that if the chain transfer process is reversible then polymerization can still possess most of the characteristics of living polymerization. A variety of terms have been used to describe polymerizations believed to involve this mechanism including “immortal polymerization”, equilibration polymerization”, “polymerization with degenerative chain transfer” and “living polymerization with reversible chain transfer”. Quirk and Lee (Polymer International 27, 359 (1992)), who recommend the last terminology, point out that the Criteria 3 and 4 mentioned above need to be modified when describing these polymerizations to encompass the fact that the total number of polymer molecules is determined by the total number of moles of transfer agent plus the number of moles of initiator.
Block copolymer syntheses by free radical polymerization in the presence of certain dithiocarbamate or xanthate derivatives as initiator-transfer agents-chain terminators (iniferters) have been described. In these examples the dithiocarbamate or xanthate derivative is used as a photochemical initiator. For a discussion of this chemistry see recent reviews [Moad et al. in Comprehensive Polymer Science; Pergamon: London, vol 3, p 141 (1989)]. The dithiocarbamates (for example, benzyl dithiocarbanate) have very low transfer constants (<<0.1) and are ineffective in the context of the current invention. Greszta et al. (Macromolecules, 27, 638 (1994)) have described the application of chain transfer chemistry in living radical polymerization and have proposed and rejected the use of dithiocarbamates in this context because of the low transfer constant and the problem of side reactions. JP 04198303 A2 discloses polymerization in the presence of triarylmethyl dithiocarboxylates of the following structure
as initiators of polymerization to yield block polymers which may have low polydispersity (all examples have Mw/Mn3 1.4). These compounds have a very weak carbon-sulfur bond that cleaves under polymerization conditions to give a stable triarylmethyl radical and a thiocarbonylthiyl radical. The product triarylmethyl radical is known to be a poor initiator of radical polymerization. They are thus ineffective in the context of this invention.Rizzardo et al. (Macromol. Symp. 98, 101 (1995)) review polymerization in the presence of addition-fragmentation chain transfer agents but do not mention the possibility of low polydispersity products.Polymers or oligomers of the following structure are known as macromonomers.

These macromonomers which are addition-fragmentation chain transfer agents are disclosed in J Macromol. Sci.-Chem. A23, 839 (1986) and International Patent publications WO 93/22351 and WO 93/22355. Free radical polymerization with living characteristics utilizing these macromonomers as chain transfer agents is disclosed in International Patent Application PCT/US95/14428. The process of this invention has the advantages of compatibility with a wide range of monomers and reaction conditions and will give good control over molecular weight, molecular weight distribution, i.e., polydispersity, and polymer architecture.